I apologise. The relevant post is https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=885777#22:
"Uh, wait... the mailing list post from October 2017 announcing the deprecation and removal of libgnome and libgnomeui and other related libraries didn't even list libgnomecanvas among the libraries to be removed, or xournal among the affected packages -- perhaps showing awareness that libgnomecanvas is in fact not related to libgnome in any way. Could it be that someone got overzealous and decided to remove libgnomecanvas for no good reason? Perhaps that can still be undone?"
